In recent years, nitride semiconductors such as GaN have been receiving attention as materials for high-withstand-voltage high-speed devices. Examples of the high-withstand-voltage high-speed devices include field-effect transistors (FETs), in particular, high-electron-mobility transistors (HEMTs). In the case of the manufacturing a GaN-based HEMT, a nitride layer, for example, a GaN layer, an AlGaN layer, or an AlN layer, is grown over a SiC substrate, a sapphire substrate, a GaN substrate, or the like.
A substrate for a compound semiconductor device including a nitride semiconductor such as a GaN-based HEMT is required to achieve satisfactory lattice matching to a nitride semiconductor layer such as a GaN layer and have high heat-dissipating properties. Furthermore, in the case of a structure in which electrodes are arranged over front and back surfaces of the substrate, i.e., in the case of a vertical structure, the substrate may be required to have conductivity. However, a sapphire substrate has low heat-dissipating properties and low conductivity. A SiC substrate and a GaN substrate exhibit high heat-dissipating properties but are very expensive.
Thus, a method including forming a nitride semiconductor layer using a substrate having satisfactory lattice matching to the nitride semiconductor layer, removing the substrate, and bonding a substrate having satisfactory heat-dissipating properties or a substrate having satisfactory heat-dissipating properties and conductivity has been studied. An example of a method for removing the substrate is a method including forming a sacrificial layer and an etching stopper layer on the substrate in advance, forming an element and so forth on the etching stopper layer, removing the sacrificial layer by photoelectrochemical etching, and separating the substrate from the etching stopper layer, the element, and so forth. The etching selection ratio of the sacrificial layer to the etching stopper layer and the level of the lattice matching of the etching stopper layer to the substrate are determined by materials thereof.
However, an increase in the etching selection ratio of the sacrificial layer to the etching stopper layer fails to form a thick etching stopper layer having satisfactory lattice matching to the substrate. Furthermore, the formation of a thick etching stopper layer having satisfactory lattice matching to the substrate reduces the etching selection ratio of the sacrificial layer to the etching stopper layer. It is thus difficult to appropriately remove the substrate.
The related art is described in the following references: Japanese Laid-open Patent Publication Nos. 2003-218087 and 2006-80274; Grenko J A, Reynolds Jr C L, Schlesser R, Bachmann K, Rietmeier Z, Davis R F and Sitar Z, “Selective Etching of GaN from AlGaN/GaN and AlN/GaN Structures”, MRS Internet J. Nitride Semicond. Res. 9 5 (2004); and Gao Y, Stonas A R, Ben-Yaacov I, Mishra U, DenBaars S P and Hu E L, “AlGaN/GaN current aperture vertical electron transistors fabricated by photoelectrochemical wet etching”, Electron. Lett. Vol. 39 No. 1 148-149 (2003).